The present invention relates to a color image forming apparatus for use in a copier, printer, printing apparatus, etc., to which an electrophotographic method, an electrostatic printing method, or the like, is applied. Specifically, the present invention relates to a digital type color image forming apparatus such as a digital type color copier, a color printer, etc., for which a highly accurate writing means using a laser beam, or the like, is required.
Generally, in a digital type color image forming apparatus using a transfer type electrophotographic method, a rotary drum type photoreceptor, around which a photoconductive photoreceptor layer is provided, is used. In the apparatus, optical scanning is conducted on the surface of the above-described photoreceptor layer by laser beams so that an electrostatic latent image is formed, and the latent image is developed into an image by toners, while the photoreceptor is being rotated. Further, in the apparatus, the toner image thus obtained is transferred onto a recording sheet. On the peripheral surface of the photoreceptor, from which the image has been transferred onto the recording sheet, residual toners which adhered onto the surface are separated and cleaned from the surface by a cleaning member, and the electrostatic latent image is formed again. Then, in the apparatus, a new image is transferred onto a recording sheet.
Specifically, in an color image forming apparatus in which a multi-color image is formed by composition of monochrome images, a plurality of developing units, in which color toners are respectively accommodated, are arranged around the photoreceptor. A latent image corresponding to each color, formed for each rotation of the photoreceptor, is noncontact-developed into a toner image, and a multi-color toner image is formed when the photoreceptor is rotated plural times. This multi-color toner image is transferred onto a recording sheet. After that, in the same way as the general image forming apparatus, the residual toner on the photoreceptor surface is separated from the surface and removed by the cleaning member. The cleaning operation is carried out when the cleaning member, composed of a blade member, or the like, comes into pressure-contact (or contact) with the image forming surface of the photoreceptor. This operation, therefore, can not be carried out during toner image formation, or before the transfer operation. Accordingly, the cleaning member is held at a position separated from the photoreceptor surface during the above-described operations. A cleaning mechanism is structured in such a manner that pressure-contact of the blade member is started just before the leading edge of the transferred toner image comes to the cleaning position, and the pressure-contact is released just after the trailing edge of the toner image has passed.
As the cleaning member which is controlled in such a manner that the cleaning member comes into pressure-contact with and is released from the photoreceptor surface in the relationship with the image forming process, there is, for example, a blade cleaning method. In this method, toner which adhered onto the peripheral surface of the photoreceptor is forcibly separated from the surface by the edge of the blade member which comes into pressure-contact with the peripheral surface of the photoreceptor.
In the cleaning apparatus using such the blade method, a multi-color toner image, which has not yet been completely formed, passes a position opposed to the cleaning apparatus, when a multi-color toner image is formed on the photoreceptor. Accordingly, the blade is separated from the photoreceptor surface, and after the multi-color toner image has been completely formed and transferred, the blade comes into pressure-contact with the photoreceptor surface and removes any residual toner.
When the blade comes into pressure-contact with and is released from the photoreceptor surface, the photoreceptor receives a load against its rotation, causing variation of the rotational torque and nonuniform rotational speed. The following are other image forming processes which become the factors of load fluctuation causing the nonuniform rotational speed of the photoreceptor drum.
(1) Pressure-contact or release of the transfer apparatus such as the transfer belt and transfer roller.
(2) Start and stoppage of the drive of the developing unit.
(3) Pressure-contact or release of the process unit, in which the developing unit, the transfer apparatus and the cleaning apparatus are integrally included.
This nonuniform rotation of the photoreceptor causes registration failure of the multi-color images and prevents sharp multi-color image formation. More specifically, when the photoreceptor causes the nonuniform rotation, nonuniform distance is caused between subsidiary scanning lines, resulting in the distance between writing lines in the subsidiary scanning direction becoming larger and/or smaller. As a result, in the image quality, density variation phenomenon of the image density, which is called an uneven image pitch or jittering, is caused, resulting in lowering of the image quality. Recently, this phenomenon is greatly emphasized as requirements of higher image quality in the market becomes greater. This phenomenon has become a large problem as the writing density of the image is increased, or as the accuracy of reproducibility of the development is increased when small diameter toners of 5.0 through 9.0 .mu.m, the diameter of which is smaller than that of conventional toners, are adopted in the apparatus.
Accordingly, it is a major problem to quickly control the speed variation due to the load, and to form an electrostatic latent image by an image writing means by highly accurate optical scanning, and by a highly accurate position control of the scanned surface.
Conventionally, in view of such problems, a large sized and heavy weighted flywheel is attached to the photoreceptor so that more uniform rotation can be maintained. On the other hand, conventionally, improvement of the optical scanning apparatus, in which the nonuniform distance between scanning lines (uneven pitch) in the subsidiary scanning direction is compensated for when optical beams scan the photoreceptor surface, has been proposed, for example, in Japanese Patent Publication Open to Public Inspection Nos. 15221/1984, and 40398/1993. Due to the contents of both patent publications, the speed of the scanned surface is detected, and the nonuniform distance between scanning lines is prevented when the optical scanning position of optical beams is changed, in cases where the speed variation occurs.
However, in the case where a flywheel is used, a relatively large mass is necessary to obtain sufficient rotational stability. When the flywheel is attached to the photoreceptor, the weight of the apparatus becomes larger, and further, a framework to support the flywheel having such a large mass is necessary. Accordingly, a more complicated large mechanism to support the large weight is necessary.
On the other hand, in attempts of the optical scanning apparatus, since the technology proposed in Japanese Patent Publication Open to Public Inspection No. 15221/1984 has the aim to mechanically operate an optical path deflecting apparatus, it is difficult to quickly and accurately respond to uneven pitches of several .mu.m, and further, the optical scanning apparatus itself can not entirely cope with vibrations which are generated in the apparatus main body or propagated from the outside of the apparatus. Further, since the technology disclosed in Japanese Patent Publication Open to Public Inspection No. 40398/1993 aims to deflect the optical path by an electric optical element, a high speed response can be realized. However, the optical scanning apparatus itself can not entirely cope with vibrations generated in the main body of the apparatus or propagated from the outside of the apparatus in the same way as the technology disclosed in the foregoing patent publication, and further, it can not cope with all vibrations relating to the root causes of the uneven pitches.